Download Chapter 3 Biology of Flowering Plants

BOT 3015L (Outlaw/Sherdan/Aghoram); Page 1 of 6
Chapter 3
Biology of Flowering Plants: Reproduction
Flowers and Pollination
Objectives
Angiosperms. Understand the distinguishing characteristics of angiosperms. Know the
differences pertaining to floral structure between and examples of the two major groups
of angiosperms, the monocots and dicots.
Flowers. Besides identifying parts of a flower, understand the relationship between
structure and function for the parts of a flower. Know the variations and terminology that
exist in flower structure (e.g. presence or absence of different reproductive parts in the
same flower, in different flowers on the same plant, and/or on different plants; symmetry;
ovary placement; inflorescences) and how these variations relate to pollination success
and evolution.
Pollination. Understand the process of pollination and the importance of the process for
completion of the plant life cycle. Be able to give examples of modes of pollination,
explain how these correlate with floral structure, and explain some of the advantages and
disadvantages for these strategies.
Introduction to Flowering Plants1
The angiosperms2, flowering plants, are the largest and most diverse group of vascular plants (we
will study the other category of plants, the non-vascular plants, later in the course). Angiosperms
constitute most of the visible vegetation on earth and most of the plants that you are familiar with
are angiosperms. Organisms in the phylum Anthophyta (angiosperms), the largest phylum of
photosynthetic organisms, range in size from the 300-foot Eucalyptus tree of Australia3 to the
aquatic duckweed 4 (family Lamnacieae) of only a few millimeters (imagine how small the
flowers are of a plant that is only one millimeter). Primary distinguishing characteristics of
angiosperms include the presence of flowers, fruits (seed(s) enclosed in a vessel or carpel, the
basic unit of the pistil), and double fertilization5 (one to produce the endosperm and the other to
form the zygote).
Monocots and Dicots. The two largest classes of angiosperms are the Monocotyledones
(monocots) with at least 90,000 species and the Eudicotyledones (eudicots or commonly dicots)
with at least 200,000 species 6. Some familiar monocots include grasses (e.g. lawn, maize,
wheat), lilies, orchids, irises, cattails, and palms. A few familiar dicots, both herbaceous
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pp. 434-435
p. 436. This name comes from two Greek words, angion (vessel), and sperma (seed). Angiosperms are
plants with seeds that are born in vessels (e.g. the pod, or fruit, of peas) .
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Fig. 19-1
4
Fig. 19-2
5
p. 446 and Fig. 19-22 and a subject of the next chapter of the lab
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p. 435-436, Fig. 19-3, and Fig. 19-4
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(non-woody) and woody (flowering trees and shrubs), include peanuts and the related legumes,
azaleas, blueberries, strawberries, oak, crape myrtle, rose, carrot, and dill. There are several
features that can be used to differentiate monocots and dicots7 and include such traits as number
of cotyledons (e.g. mono (one) cot (cotyledon)), function of cotyledons, presence or absence of
endosperm at seed maturity, number of flower parts, root structure, presence or absence of
secondary growth in the shoot, vascular bundles arrangement, and leaf venation. Monocots and
dicots will be contrasted throughout this course.
In your lab notebook, create a table to contrast monocots and dicots. Include at least the
characteristics mentioned above. Likely, you will be adding to this table as the semester
progresses. Utilize your textbook, the BOT 3015 lecture notes, and any other reliable resources
available to you.
A guided, interactive tour of a flower. The presence of a flower is a major distinguishing
characteristic of angiosperms. Taxonomists use details of floral structure to identify species;
thus, one reason it is important to learn the terminology associated with floral structure. In
addition, a deeper understanding of the structure leads to an understanding of the function of a
flower. The parts of a flower8 are, anatomically, modified leaves born on shoots that have
evolved as reproductive parts of angiosperms.
Specimen 1: Flower9 dissection
1. When obtaining a flower, be sure to cut below the receptacle, the part of the flower stalk to
which the floral parts are attached.
2. Notice how the floral parts are arranged around the vertical axis of the flower. One floral
evolutionary trend is for the parts to move from a spiral arrangement to a whorled
arrangement at separate levels.
3. The most recognizable parts of most flowers are the petals; however, the outermost parts of a
flower are the sepals. The sepals are collectively called the calyx and are what encase the
flower as a bud. Many times they are green and leaf-like, but in some cases look very much
like petals, in which case they are called petaloid or are called tepals.
4. Just inside the sepals, you will find the petals, collectively called the corolla. Together the
infertile calyx and the corolla are called the perianth.
5. Count and note the number of sepals and petals.
6. Carefully, to keep the other parts of the flowers intact, dissect and keep intact the sepals and a
few petals from the flower.
7. In your notebook, create a dissection press with unlined paper. The first page of the flower
dissection press will be blank and will serve as a cover. Keep one sepal and tape one sepal to
the second page at the top of the page (see diagram step 9). Do the same with petals, taping a
couple of them on the bottom half of the page. Label them.
8. Observe the sepal and petal that you saved under the dissecting microscope. Under the taped,
lived specimens, describe what you see under the dissecting microscope.
9. You will be setting up a third page of your press, similar to the page with the sepals and
petals, for the reproductive parts of the flower, the stamens and carpel. After dissecting away
the perianth, the entire structures of the reproductive structures are revealed. First look at the
pollen-bearing stamens, collectively called the androecium (andro (man) ecium (house)), that
7
Table 19-1 and table in Angiosperm Anatomy and Selected Aspects of Physiology of Outlaw’s BOT 3015
lecture notes available at http://www.southernmatters.com
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pp. 436-437
9
As you are dissecting and observing this specimen, use Fig. 19-6 as a reference
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surround the pistil that is in the middle. The stamens are also called microsporophylls
(microspore-producing modified leaves). In most angiosperms, the stamen is made up of a
two-lobed anther, containing microsporangia (pollen sacs), atop a filament. Count and note
the number of stamens.
10. Dissect a few of the stamens from the flower, keep one and tape the others onto the top of a
third page of the flower-dissection press in your notebook and label them.
11. Observe the stamen that you kept under the dissecting scope describe (including function)
under the taped specimen. While you are looking, dissect into the anther to find the pollen.
12. You are left with the carpel (when fused or single, also termed pistil), collectively known as
the gynoceium (gyno (woman) ecium (house)). The leaf-like megasporophyll encloses one or
more ovules. The tip of the carpel (furthest from the receptacle) is the stigma that receives
pollen. The pollen grows a pollen tube that travels down the style, which is usually long, of
the carpel until it reaches the ovules that are inside the lower part, the ovary, of the carpel. It
is within the ovule that the megagametophyte (“female” gametophyte, derived from mitotic
cell division of spores (1N and produced from meiosis)), is found. We will look closer at the
gametophyte in the next lab.
13. Under the dissecting scope, carefully slice the ovary open to observe the ovules inside. After
fertilization, the ovules develop into seeds. When carpels fuse, the ovary is generally, but not
always, partitioned into chambers called locules. The portion of the ovary from which the
ovules originate and remain attached is the placenta. The arrangement of the placentae,
known as placentation 10, varies and is a way for taxonomists to distinguish different species.
Leaving a place to tape the opened carpel to the middle of page three of your press (under the
stamen section), draw the carpel, details as under the dissecting scope, label (pistil, stigma,
style, ovary, and ovules), and describe (including function) the opened carpel.
14. Finally, tape the opened carpel onto the lower half of page three.
Variations in flowers and trends in floral evolution11
1. Evolution tends to favor reductions in the number of floral parts 12. For example, the
Gladiolus flower that you just dissected has both carpels and stamens, this is a perfect flower;
whereas, an imperfect flower is missing one of these reproductive parts. Imperfect flowers
may be either staminate, having only stamens and no pistil, or pistillate, having no stamens.
Of course, for species with imperfect flowers to reproduce, both staminate and pistillate
flowers must exist for that species. For monoecious (mono (one) ecious (house)) species,
such as maize and oak, staminate and pistillate flowers exist on the same plant. For dioecious
(di (two) and ecious (house)), such as Cannibas sativa and willow, pistillate and staminate
flowers do not exist on the same plant body, but on different plant bodies.
2. As mentioned before, a cyclic arrangement of parts is favored evolutionarily over a spiral
arrangement.
10
p. 438 and Fig. 19-9
p. 458
12
p. 438
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3. Evolution tends to favor fused floral parts over free parts. As discussed during the flower
dissection, carpels are often fused. One can also find examples of fused petals, stamens, and
even fusion of stamens and pistils.
4. Floral evolution has tended to favor bilateral symmetry (irregularity) over radial symmetry
(regularity). Flowers have evolved ornately shaped petals to attract pollinators or as guides
and landing pads for pollinators. Some intermediates in regularity exist such as radially
symmetric shape, but bilaterally symmetric color.
5. There are also evolutionary trends in the insertion of floral parts along the floral axis. In
some cases, the sepals, petals, and stamens arise from below the ovary, which is termed
superior. When the ovary is below the insertion points of the sepals, petals, and stamens, it is
termed inferior. The evolutionary trend is toward an inferior ovary. Of course, there are
intermediates between these two conditions.
In your lab notebook answer the following questions (all answers in your lab notebook
should be incomplete sentences) about the flower that you dissected.
1. Is it a monocot or dicot and what determining characteristics did you use?
2. Is the ovary superior or inferior?
Specimens 2 and 3: More flowers
1.
2.
3.
4.
5.
6.
7.
8.
9.
Obtain two of each of two more flowers.
Count the number of sepals, petals, stamens, and pistils.
Dissect away a couple of petals and/or sepals to reveal the ovary.
Draw and label the flower with the intact ovary.
Make a longitudinal section of one of each kind of flower, including the ovary, draw and
label (sepals, petals, stamens, stigma(s), style, ovary, ovules).
Make a cross-section of the ovary of one of each kind of flower, draw and label (ovary,
ovules).
To your dissection press (new page), add sepals, petals, stamens, and ovaries from these
flowers.
List the traits of each flower that are advanced.
List the traits of each flower that are primitive.
Inflorescences13
Many plants produce their flowers in clusters called inflorescences. Although there are many
inflorescence arrangements, one of the most advanced kinds of inflorescences is produced by
members of the composite family such as sunflowers, daisies, and dandelions. What appear to be
petals on the edge of a sunflower or daisy are actually individual flowers called ray flowers. Ray
flowers are bilaterally symmetrical, pistillate flowers with one enlarged petal that, together with
the other ray flowers, attract pollinators to the inflorescence. The central part of the inflorescence
usually consists of numerous, small disk flowers, each of which is radially symmetrical perfect
flower with tiny petals.
Specimen 4: Inflorescence
1. Observe the inflorescence with your naked eye and identify the ray and disc flowers.
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Figs. 19-7 and 19-8, pp. 458-459 and Fig. 20-9
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2. Carefully tease out a ray flower and a disc flower. Observe them under the dissecting
microscope. Try to find the reproductive structures in each of the flowers.
3. Tape a disc and a ray flower into your press and, beside each, describe the reproductive
structures found in each.
Pollination and the co-evolution of angiosperms and insects
Pollination is the process by which pollen grains 14, which contains the microgametophyte, are
carried over to the megagametophyte in the carpel or pistil. This process is imperative for
completion of the plant life cycle. Plants are sessile; thus, they must depend on external factors to
carry out pollination. Land plants accomplish this in two different ways, by wind and by biotic
vectors.
Wind facilitated pollination15 is the major mechanism of pollen transfer in non-flowering seed
plants, such as pine, but is also present in angiosperms, such as oak and grasses. Wind
pollination can be an efficient means of uniting micro- and megagametophytes when there is a
large population of one species in a small area, such as a pine forest or a field of grass. Instead of
investing energy into producing showy flowers to attract biotic vectors, wind-pollinated plants
invest energy into making large amounts of pollen.
Some flowering plants have evolved flowers that attract biotic vectors such as insects, birds, and
some mammals16. Flowers can attract biotic vectors by site, smell, and/or rewards such as nectar
or pollen (some flowers even mimic, with petal structure and color, a mate of the biotic vector);
thus, flowers and their biotic vectors have co-evolved in a mutually beneficial manner. In many
cases, this relationship is very specific; meaning that a particular species of plant is only fertilized
by a particular animal species. Flowers have evolved specific structures, colors, smells, and
rewards that make them most attractive for their vectors, resulting in a fascinating diversity and
complexity of flowers. In addition, flowers have evolved specific placement of reproductive
parts to maximize successful pollination. Animal vectors evolve structures that enable them to
successfully obtain the reward such as the long proboscis of some moths and the long, slender
beak of the hummingbird. Thus, the variations in flower morphology that you observe are not
random, but are the results of millions of years of evolution during which increasingly complex
and specialized reproductive strategies developed.
The earliest angiosperms were insect pollinated, probably by beetles, but they retained the
primitive tendency toward having many reproductive structures. Magnolia trees, which are
considered one of the most primitive angiosperms, are pollinated by beetles and have superior
ovaries in radially symmetrical flowers with many floral parts. Beetles are rewarded during
pollination with food, they eat the flower parts, but do not destroy all the fertilized ovules. Beetle
pollination is more efficient than wind pollination for this species, but clearly, leaves much to be
desired for the plant.
Bees are the main biotic vectors for pollination, but flies, wasps, butterflies, moths, birds, bats,
and more are pollination vectors. Often, the association of a flower and its pollinator is quite
specific. Perhaps, the most extreme example is that of the wasp-pollinated orchid Ophrys.
Orchids are among the most advanced angiosperms; the ovary is inferior and stamens, petals, and
pistils are fused. In Ophrys, the petals closely resemble the female of a certain species of wasp.
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pp. 442-444 and Figs. 19-14, 19-15, and 19-16.
p. 463
16
pp. 460-462
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This resemblance is so strong that male wasps attempt to copulate with the flowers and in so
doing, carry pollen from flower to flower. Some plants spend large amounts of energy to attract
insects. The arum lily, generates heat that causes the evaporation of volatile chemicals that attract
insects to the flower.
Video: Pollination Biology
In addition to the video, you will certainly see pollination taking place around you.
Review Questions
In addition to the questions and writings found in text.
1. What are the functions of a flower and how are these functions important for the survival of a
species?
2. For each of perfect and imperfect flowers, give at least one advantage that each has over the
other.
3. Based on the function of flowers, why do you think flower structure is diverse across plant
families and sometimes species, but evolutionarily conserved within species?
4. Why do you think there is an evolutionary trend toward (i.e. what are the advantages to)
reduced numbers of floral parts?
5. Describe at least one advantage of inflorescences for reproductive success.
6. Some angiosperms invest energy in ornate flowers (e.g. orchids and sunflowers) and others
(e.g. grasses and oaks) do not. Give one possible explanation for how they are both
successful despite this difference.